Investment casting mold and method of manufacture

ABSTRACT

A process for rapidly forming a ceramic shell mold on an expendable pattern is disclosed. The process entails use of refractory slurries which include a large particle size colloidal silica sol binder. The colloidal silica sol binder has an average particle size of about 40 nanometers, i.e., about 3-4 times larger than colloidal silica sol binders heretofore employed in manufacture of ceramic shell molds. The use of the large particle sols yields unfired ceramic shell molds which have about 40% to about 70% greater unfired strengths compared to ceramic shells made with prior art small particle size silica sols. Prime coats and refractory back-up coats which use the large particle size sol dry about 30% to about 40% faster than prime coats and back up coats which employ the smaller particle size silica sols of the prior art.

This application is a continuation of application U.S. Ser. No.09/105,782 filed Jun. 26, 1998, now U.S. Pat. No. 6,000,457. ThisApplication also claims benefit to provisional Application 60/074,366Feb. 11, 1998.

FIELD OF THE INVENTION

This invention relates to improved methods and compositions forinvestment casting technology.

BACKGROUND OF THE INVENTION

Investment casting by the lost wax process can be traced to ancientEgypt and China. The process as practiced today, however, is arelatively new technology dating to the 1930's and represents a rapidlygrowing business and science. Investment casting technology simplifiesproduction of complex metal shapes by casting molten metal intoexpendable ceramic shell molds formed around disposable wax patternswhich duplicate the desired metal shape. “Precision Investment Casting”,i.e., PIC, is the term in the art that refers to this technology.

The conventional PIC process employs six major steps:

(1) Pattern preparation.

A disposable positive pattern of the desired metal casting is made froma thermoplastic material such as wax that will melt, vaporize or burncompletely so as not to leave contaminating residues in the de-waxedceramic shell mold. The positive pattern is prepared by injecting thethermoplastic material into a negative, segmented, metal die or “tool”designed to produce patterns of the shape, dimension and surface finishrequired for the metal casting. Single or multiple patterns can beassembled by fusing them to a disposable wax “sprue system” that feedsmolten metal to fill the shell mold;

(2) Shell mold construction by:

(a) dipping the pattern assembly into a refractory slurry having fineparticulate refractory grain in an aqueous solution of alkali stabilizedcolloidal silica binder to define a coating of refractory material onthe pattern;

(b) contacting the refractory coating with coarse dry particulaterefractory grain or “stucco” to define a stucco coating, and

(c) air drying to define a “green” air dried insoluble bonded coating.These process steps can be repeated to build by successive coats a“green”, air dried shell mold of the desired thickness.

(3) Dewaxing—The disposable wax pattern is removed from the “green” airdried shell mold by steam autoclaving, plunging the green shell moldinto a flash de-waxing furnace heated to 1000° F.-1900° F., or by anyother method which rapidly heats and liquefies the wax so that excessivepressure build-up does not crack the shell mold.

(4) Furnacing—The de-waxed shell mold is heated at about 1600° F.-2000°F. to remove volatile residues and form stable ceramic bonds in theshell mold.

(5) Pouring—The heated shell mold is recovered from the furnace andpositioned to receive molten metal. The metal may be melted by gas,indirect arc, or induction heating. The molten metal may be cast in airor in a vacuum chamber. The molten metal may be poured statically orcentrifugally, and from a ladle or a direct melting crucible. The moltenmetal is cooled to produce a solidified metal casting in the mold.

(6) Casting recovery—The shell molds having solidified metal castingstherein are broken apart and the metal castings are separated from theceramic shell material. The castings can be separated from the spruesystem by sawing or cutting with abrasive disks. The castings can becleaned by tumbling, shot or grit blasting.

Binders used in the refractory slurries affect the shell buildingprocess and ultimate shell mold quality. Binders should be chemicallystable to ensure long service from a refractory slurry used forrepetitive dip coats. Binders also should form insoluble bonds with therefractory grains during air drying to permit redipping of the patternas well as to permit removal of the pattern during furnacing. Thestabilized ceramic bonds produced in the shell during furnacing moldmust also have adequate fired strength and refractoriness so as towithstand casting of molten metal.

Standard refractory slurry binders which have been employed inmanufacture of ceramic shell molds include hydrolyzed ethyl silicatesand small particle size sodium stabilized colloidal silicas having anaverage particle size of about 8-14 nanometer. The latter includesalkaline aqueous dispersions of colloidal silica stabilized with sodiumhydroxide which are non-flammable and have low toxicity. The former isacid stabilized with sulfuric or hydrochloric acid added duringhydrolysis to form colloidal silica in situ. The former, however,employs flammable, toxic alcohol solutions to maintain solubility. Theethyl silicate binders, however, permit faster drying and use lowerlevels of flux promoting sodium oxide.

In the conventional process for making ceramic shell molds, the intervalrequired for drying between coats may vary from 30 minutes forrefractory prime coats to 8 hours or more for back-up coats depending onmold complexity and shell wall thickness. Completed shell molds areusually air dried an additional 24 hours or more to assure adequategreen strength for pattern removal. This dependence on air drying forshell mold quality accounts for a major portion of production time,contributes to high production costs and is a serious shortcoming.

Because of this shortcoming, numerous efforts have been made to shortenor eliminate the time interval required for drying between coats byusing chemical methods to rapidly set the refractory slurry binder.These chemical methods have broadened the choice of refractory slurrybinder candidates beyond hydrolyzed ethyl silicate and sodium stabilizedcolloidal silica to include ionic alkali metal silicates, and acidstable alumina modified colloidal silica. These prior art chemicalmethods include:

(1) Use of a gaseous gelling agent to gel set a slurry binder system.

U.S. Pat. No. 2,829,060 teaches the use of carbon dioxide to gel set anammonia modified sodium silicate slurry binder system.

W. Jones, in a technical paper presented to the Investment CastingInstitute in October of 1979, disclosed the use of carbon dioxide oracidic alumina solutions to set alkaline silicate binder slurries.Alkaline silicate binder slurries, however, can cause undesirablefluxing at high temperatures.

U.S. Pat. No. 3,455,368 teaches the use of ammonia gas to gel set ahydrolyzed ethyl silicate or acidified colloidal silica binder system.Ammonia gas, however, is toxic.

U.S. Pat. No. 3,396,775 teaches the use of volatile organic gases to gelset a hydrolyzed ethyl silicate slurry binder system. Volatile organicgases, however, present a ventilation problem that contributes to pooracceptance in the foundry.

(2) Use of two interacting slurry binder systems to gel set one anotherwhen applied as alternating coats.

U.S. Pat. No. 2,806,270 teaches the use of:

1) nitric acid acidified sodium silicate slurry to gel set an alkalinesodium silicate slurry;

2) a phosphoric acid acidified potassium silicate slurry system to gelset any of:

(a) an alkaline potassium silicate slurry,

(b) an alkaline piperidine modified ethyl silicate slurry, and (c) analkaline mono-ethanolamine modified ethyl silicate slurry system;

3) an acidic ethyl silicate slurry to gel set any of:

(a) an alkaline potassium silicate slurry,

(b) an alkaline piperidine modified ethyl silicate slurry, and

(c) an alkaline mono-ethanolamine modified ethyl silicate binder system.

U.S. Pat. No. 3,751,276 and U.S. Pat. No. 3,878,034 teach the use of anacid stable alumina modified colloidal silica slurry binder system togel set either an alkali stable ionic silicate binder slurry system oran alkali stabilized colloidal silica binder slurry system. The use oftwo interacting slurry binder systems, however, requires a change inconventional shell making procedure.

(3) Use of a chemically treated stucco grain to gel set a binder slurrysystem.

Dootz, Craig and Payton in Journal Prosthetic Dentistry Vol. 17, No. 5,pages 464-471, May 1967 describe the use of monoammonium phosphate andmagnesium oxide treated stucco to gel a sodium silicate binder slurrysystem. This approach, however, suffers the disadvantage that itseffectiveness degrades over time and can contaminate the refractorybinder slurry.

(4) Use of a gelling agent solution to gel set a binder slurry system.

U.S. Pat. No. 3,748,157 teaches the use of a basic aluminum salt settingagent solution to gel set

1) a sodium stabilized negative sol colloidal silica binder slurry, and

2) an alkaline ionic silicate slurry binder system.

Although these methods of the art have varying degrees of usefulness inpreparing ceramic shell molds for use in PIC, they nevertheless requiremultiple catalyzation steps or substantial time intervals betweensuccessive coatings of refractory slurry materials. A need thereforeexists for materials and methods which rapidly form ceramic shell molds.

SUMMARY OF THE INVENTION

The invention relates to a process for rapidly forming a ceramic shellmold on a disposable support member, and to the ceramic shell moldsobtained thereby. The process employs a large particle size colloidalsilica sol that has an average particle size of about 40 nanometer, awide particle size range of about 6 nm to about 190 nm, and a standarddeviation of about 20 nm. The large particle size sol which preferablyis employed is available under the tradename Megasol™ from WesbondCorp., Wilmington, Del. MegasoI™ has an average particle size of about40 nanometer, a particle size range of about 6 nm to about 190 nm, astandard deviation of particle sizes of about 20 nm, and a sodiumcontent of about 0.22% vs. sodium contents of about 0.4 to 0.6% of priorart colloidal silica sols.

The process of the invention offers a number of advantages for themanufacture of ceramic shell molds over the above described prior artprocesses. For example, use of aqueous Megasol™ colloidal silica solenables manufacture of green ceramic shell molds which have about 40% toabout 70% greater unfired strengths compared to green ceramic shellsmade with prior art silica sols which have much smaller ranges ofparticle sizes.

Another advantage of the invention is that refractory slurrycompositions which employ Megasol™ can accommodate a wide range of shellmold thermal-expansions. A further advantage is that refractory slurrycompositions which employ Megasol™ have a colloidal silica solidscontent of about 40% to about 50% in the refractory slurry. These solidscontents are much greater than the colloidal silica solids contents ofabout 22% to about 27% achieved in the refractory slurries which useConventional, small particle size silica sol binders. The highercolloidal silica solids content in refractory slurries which employMegasol™ advantageously enables more rapid drying of both refractoryprime Boats and refractory back-up coats.

Use of Megasol™ in at least one of the refractory prime coat slurriesand refractory back-up coat slurries, preferably both slurries, yieldsincreased stability of the slurries as well as higher strength ceramicshell molds. The invention advantageously eliminates the common industrypractice of using a polymer in refractory slurries or using a polymerenhanced binder in refractory slurries. Elimination of polymersadvantageously overcomes the prior art problem of manufacture of ceramicshell molds which have low fired modulus of rupture due to porositygenerated when the polymer is burned out during furnacing. Eliminationof polymers also overcomes the prior art problem of destabilization ofrefractory slurries over time as well as problems associated withquality control of the refractory slurries.

Prime coats and back-up coats which employ Megasol™ also dry about 30%to 40% faster than prime coats and back up coats which employ thesmaller particle size colloidal silica sols of the prior art. Thisenables shorter drying times which reduces the cost of manufacture ofthe shells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a positive disposable pattern 1 of a desired metalcasting.

FIG. 2 is an isometric view of a green shell 10 prior to removal ofpattern 1.

FIG. 3 is an isometric view of a dewaxed, dried green ceramic shell 20.

DETAILED DESCRIPTION OF THE INVENTION

Refractory Grains A wide variety of refractory grains may be used withMegasol™ in refractory prime coat slurries as well as in refractoryback-up coat slurries. Examples of these refractory grains include butare not limited to mullite, calcined china clay and other aluminasilicates, vitreous and crystalline silica, alumina, zircon andchromite. The refractory grains preferably are free of ioniccontaminates in amounts that can contribute to instability of therefractory grains and to thermally induced phase changes which can occurduring metal casting. As is known in the art, refractory grains whichare free from contaminates in Amounts that can contribute to instabilityof the refractory grains can be produced by purification with or withoutcalcining.

Preparation of Refractory Slurries

Refractory prime coat slurries and refractory back-up coat slurriesutilize large particle size silica sol binders such Megasol™ withrefractory grain in amounts sufficient to have a desired viscosity foruse in the shell dipping process. Preferably, Megasol™ having a specificsurface area of about 68 m²/gm, an average particle size of about 40nanometer, a particle size range of about 6 nm to about 190 nm, astandard deviation of particle sizes of about 20 nm, and a sodiumcontent of about 0.22% is employed. The average particle size ofMegasol™ is calculated by dividing the number 2727 by the specificsurface area. The amounts of Megasol™ and refractory grain in therefractory slurry compositions can be varied over a wide range.

Megasol™ silica sol binder has a much larger particle size range andlower specific surface area than prior art colloidal silica sol binders.Megasol™ silica sol binder may be used at a pH of about 8.0 to about10.0, preferably at a pH of about 9.0 to about 9.5. Megasol™ silica solbinder may be used at titratable Na₂O contents of about 0.02% to about0.35%, preferably about 0.1% to about 0.25%. Most preferably, Megaso™silica sol binder is used at a titratable Na₂O content of about 0.20% toabout 0.22%.

Megasol™ silica sol binders for use in the invention may have varyingsolids contents. For example, Megasol™ may be used at a solids contentof about 30% to about 50% solids content, preferably about 40 to about47% solids content. More preferably, Megasol™ is used at about 45%solids content in at least one of the refractory prime coat slurries andrefractory back-up coat slurries, most preferably in both slurries.

Refractory prime coat slurries and refractory back-up coat slurries areprepared by placing Megasol™ silica sol binder into a clean, waterrinsed mixing tank and adding refractory material while mixing. Variousmixing devices known in the art may be employed in the mixing tank.These devices include, for example, propeller type mixers, jar mills,high speed dispersion mixers, and turntable fixed blade mixers.

Refractory material is added while mixing until a desired viscosity isreached. For refractory prime coat slurries, this viscosity is typicallyabout 18-30 seconds No. 4 Zahn, preferably 20-30 sec, most preferably24-30 sec. Suitable viscosities for refractory back-up coat slurrieswhich employ Megasol™ and fused silica refractory grain are about 10-18sec. viscosity Zahn #4, preferably about 10-16 sec Zahn#4, mostpreferably about 12-15 sec Zahn #4. After additional mixing to removeentrapped air and to reach equilibrium, a final viscosity adjustment ismade by adding additional Megasol™ Colloidal silica sol binder orrefractory material. Non-ionic surfactant and anionic surfactants alsocan be added to the refractory slurries.

Shell Mold Construction

Shell mold construction begins with the application of one to threecoatings of a refractory me coat slurry that includes refractory gainsand Megasol™ to a clean, disposable pattern, preferably a wax pattern.The wax pattern preferably is formed from any filled or unfilledparaffin based investment casting grade wax, or microcrystalline wax.The wax pattern is dipped into the refractory prime coat slurry to coatthe surface of the pattern with a continuous layer of refractory primecoat slurry, drained thoroughly to remove excess slurry, and thenstuccoed with prime coat refractory stucco. The resulting prime coat(s)can have a thickness of 0.02″ to 0.2″, preferably 0.04″ to 0.2″, mostpreferably 0.04″ to 0.1″.

Different refractory slurry compositions may be used in the refractoryprime coat slurries and refractory back-up coat slurries. The specificrefractory prime coat slurries and refractory back-up coat slurries aredetermined by the ceramic shell mold characteristics desired to producea metal casting having desired dimensions and surface finish from thedisposable pattern.

The refractory prime coat slurry employs the finest sizes of refractorygrain, usually about −200 mesh and finer, down to about −325 mesh.Refractory prime coat slurries which may be employed include Megasol™together with a blend of −200 mesh fused silica and −325 mesh zirconflour. The zircon flour provides high resistance to molten metal. Thefine particle size of the zircon flour also enables production ofcastings which have smooth, detailed surface finishes. Each prime coatis stuccoed with a coarse refractory grain, typically zircon sand ofabout −20 to about 200 mesh, preferably −70 to 140 mesh.

In refractory prime coat slurries which employ Megasol™, fused silicaand zircon, the fused silica most preferably has a particle size ofabout −120 to about −200 mesh, and the zircon most preferably has aparticle size of about −325 mesh. Fused silica sizes of about −100 mesh,about −120 mesh, about −140 mesh, about −170 mesh, about −270 mesh andabout −325 mesh also may be used. Particle sizes of the zircon may be,for example, about −200 mesh, about −325 mesh and about 400 mesh.Preferably, the Zircon is about −200 mesh. Non-ionic surfactantsoptionally may be added to the refractory prime coat slurry. Aparticularly useful non-ionic surfactant which may be employed is PS9400available from Buntrock Industries, Williamsburg, Va. This surfactantcan be added to the refractory prime coat refractory slurry in an amountof up to about 0.2% based on the weight of the Megasol™ binder. Thissurfactant improves the ability of the refractory prime coat refractoryslurry to wet the wax pattern and also assists in drainage.

Refractory back-up slurries are applied to the stuccoed, prime coats toproduce back-up coats. Refractory back-up slurries employ coarserrefractory grain sizes than are used in refractory prime coat slurries.In refractory back-up slurries where fused silica is employed withMegasol™, the fused silica may have a particle size of about −80 mesh toabout −270 mesh, preferably about −100 mesh to about −200 mesh. Mostpreferably, the fused silica is about −100 mesh to about −120 mesh. Eachback-up coat is stuccoed with a coarse refractory grain to buildthickness in the shell for added strength. The refractory grains whichmay be used as stucco on the back-up coats may vary from about −10 meshto about 50 mesh, preferably about −20 mesh to about 50 mesh. Mostpreferably, these refractory grains have a size of about −30 mesh toabout 50 mesh.

Back-up coats are applied over the stuccoed prime coats until the shellreaches a desired thickness and strength. The number of back-up coatsapplied depends on the size and weight of the metal casting to be formedin the ceramic shell. A thickness of ceramic shell of about 0.20 inch to0.5 inch is sufficient for most castings. Two prime coats, and 4 to 5back-up coats typically yield a 0.25 inch thick green shell that has astrength sufficient to withstand dewaxing and furnacing.

In an alternative embodiment, a transitional stucco refractory material,preferably zircon or an alumino silicate which has a grain sizeintermediate between the fine grained prime coat stucco and the coarseback-up coat stucco, may be applied to the prime coat-stuccoedexpendable pattern prior to application of the coating of refractoryback-up slurry. The transitional stucco coat can be used to add strengthto the green shell and to minimize the possibility of delaminationbetween the final coating of prime coat slurry and the first coating ofrefractory back-up slurry.

The green shell is dried at about 60° F. to about 90° F., preferablyabout 70° F. to about 75° F. Drying may be performed under acceleratedconditions of low humidity and high temperature with rapid air movement.

The drying time between successive prime coats and back-up coats dependson the complexity of the shape of the expendable pattern. Expendablepatterns which have deep cavities where airflow is minimal take longerto dry between coats. Simple patterns which have flat sides dry faster.Prime coats and back-up coats formed from refractory slurries whichemploy Megasol™ dry about 30% to about 40% faster than industry standardrefractory slurries which use much smaller particle size colloidalsilica sol binders and which contain higher amounts of water.

Dewaxing

The green ceramic shell molds may be dewaxed by immersion into boilingwater, steam autoclaving, and flash dewaxing as is known in the art.Steam autoclaving may be performed by:

1. Using as high a steam pressure as possible, preferably about 60 psior higher, more preferably about 80-90 psi.

2. Closing and pressurizing the autoclave as rapidly as possible,preferably in less than about 15 to 20 seconds.

3. Exposing the air dried green shell to the steam for about 10 to 15minutes.

4. Slowing depressurizing the autoclave over about 30 to 60 seconds.

Flash dewaxing may be performed by plunging the air dried green shellmold into a furnace heated to about 1000° F. to about 1900° F. At thesetemperatures, the wax next to the wall of the ceramic shell rapidlymelts so that the pressure due to expansion of the wax does not crackthe ceramic shell. The ceramic shell may then be removed to a coolertemperature zone of about 200° F. to 600° F. to complete the removal ofthe wax. The melted wax can drain through a bottom opening in themelting chamber into a water bath or reservoir for recovery.

Furnacing

Furnacing entails heating the dewaxed ceramic shell mold produced aboveto about 1600° F. to about 2000° F. to remove volatile residues and toproduce a high strength ceramic shell mold by forming stable ceramicbonds through sintering. The dewaxed ceramic shell mold is held in thefurnace to attain thermal equilibrium, after which it is retrieved fromthe furnace and cast with the desired molten metal.

The invention is further described below by reference to the followingnon-limiting examples.

EXAMPLE 1

An 8 inch by ⅞ inch by ⅜ inch wax bar pattern 1 as shown in FIG. 1 isdipped into a refractory slurry of the composition shown in Table 1. Forconvenience, the same refractory slurry is used for both prime andback-up coats.

TABLE I MATERIAL AMOUNT Megasol ™¹ 1000 gm Tecosil 120F² 1500 gm Zircon325³ 400 gm PS 9400 Surfactant⁴ 2 ml ^(1.)Megasol ™ colloidal silica solbinder having 50% solids content from Wesbond Corp. ^(2.)Fused silicafrom C-E Minerals, particle size of 44-177 microns ^(3.)Calcined FloridaZircon, particle size of −325 mesh from Continental Minerals^(4.)Non-ionic surfactant available from Buntrock Industries,Williamsburg, VA. PS 9400 is a polyoxethylated decyl alcohol that has aspecific gravity of about 1.0, and a pH of about 7 to 9.

Wax pattern 1 is dipped into the refractory slurry for 5 seconds,removed, and allowed to drain for 10 seconds to form a first prime coat.Zircon sand of −70 to 140 mesh available from DuPont Corp. is applied asstucco to the first prime coat. The zircon sand stuccoed, prime coatedbar pattern is dried for one hour, and then again dipped into therefractory slurry for 5 seconds to form a second prime coat and againstuccoed with the zircon sand of −70 to 140 mesh.

Wax pattern 1 having two stuccoed prime coats then is dipped into therefractory slurry for five seconds and drained for ten seconds toprovide a first back up coat. The first refractory back-up coat then isstuccoed with Tecosil −30 mesh to 50 mesh fused silica available fromC-E Minerals. The stuccoed back-up coat then is dried for one hour. Thisis repeated to provide a total of five back-up coats stuccoed with theTecosil −30 mesh to 50 mesh fused silica. After application of eachprime coat and refractory back-up coat, vertical sides 5 of pattern 1are scraped to remove the coats and stucco. The resulting green ceramicshell 10 formed on pattern 1 having two prime coat-zircon sand stuccolayers and five back-up coat- stucco layers where the stucco is Tecosil−30 to +50 mesh fused silica from C-E Minerals Co., as shown in FIG. 2,again is dipped into the refractory slurry to provide a seal coating.The seal coating is not removed from the sides of pattern 1.

The seal coated, green ceramic shell is dried at 70-75° F. overnight.The dried, green ceramic shell is immersed in boiling water to removepattern 1. The resulting dewaxed, dried green ceramic shell 20, shown inFIG. 3, is cut in half lengthwise, and dried over night. A section ofthe green ceramic shell that measures 1 inch wide by 6 inches long by0.3 inches thick is evaluated for strength by loading a 2 inch span ofthe section to failure in flexure. The modulus of rupture (“MOR”) of thegreen ceramic shell is calculated using the formula:

R=(3WI)/(2bd ²)

where:

R=modulus of rupture in lbs/in²

W=load in pounds at which the specimen failed

l=distance (span) in inches between the center-lines of the lowerbearing edges

b=width of specimen in inches

d=depth of specimen in inches

The modulus of rupture is shown in Table 2.

A section of the green ceramic shell that measures 1 inch wide by 6inches long by 0.3 inches thick is fired at 1800F. for one hour. Thefired section then is evaluated for strength by loading a 2 inch span ofthe section to failure in flexure as described above. The modulus ofrupture (“MOR”) of the fired ceramic shell is calculated using theformula above. The results are shown in Table 2.

EXAMPLE 2

The procedure of example I is followed except that the Megasol™ isdiluted with water to provide a colloidal silica solids content of 45%.The MOR is measured as in example 1.

EXAMPLE 3

The procedure of example 1 is followed except that the Megasol™ isdiluted with water to provide a colloidal silica solids content of 40%.The MOR is measured as in example 1.

EXAMPLE 4

The procedure of example 1 is followed except that the Megasol™ isdiluted with water to provide a colloidal silica solids content of 35%.The MOR is measured as in example 1.

EXAMPLE 5

The procedure of example 1 is followed except that Mulgrain M47-22Shaving a particle size of −20+50 mesh is substituted for the −30+50 meshTecosil fused silica. Mulgrain M47-22S is available from CE Minerals Co.The MOR is measured as in example 1.

EXAMPLE 6

The procedure of example 5 is followed except that the Megasol™ isdiluted with water to provide a colloidal silica solids content of 45%.The MOR is measured as in example 1.

EXAMPLE 7

The procedure of example 5 is followed except that the Megasol™ isdiluted with water to provide a colloidal silica solids content of 40%.The MOR is measured as in example 1.

COMPARATIVE EXAMPLES 8-12 EXAMPLE 8

The procedure of example 1 is followed except that NYACOL 830 colloidalsilica sol having an average particle size of about 8 nanometer and acolloidal silica solids content of 30% is substituted for the Megasol™colloidal silica sol having 50% solids content. NYACOL 830 is availablefrom EKA Chemicals Co. The MOR is measured as in example 1.

EXAMPLE 9

The procedure of example 8 is followed except that NYACOL 830 is dilutedwith water to provide a colloidal silica solids content of 24%. The MORis measured as in example 1.

EXAMPLE 10

The procedure of example 8 is followed except that Mulgrain M47-22S of−20+50 mesh size is substituted for the −30+50 mesh Tecosil fusedsilica. The MOR is measured as in example 1.

EXAMPLE 11

The procedure of example 10 is followed except that NYACOL 830 isdiluted with water to provide a colloidal silica solids content of 27%.The MOR is measured as in example 1.

EXAMPLE 12

The procedure of example 10 is followed except that NYACOL 830 isdiluted with water to provide a colloidal silica solids content of 24%.The MOR is measured as in example 1.

To illustrate the reduced drying times achievable with use of Megasol™,the total drying times for the five back-up coats applied in examples 1and 8 are compared. Drying time is measured using a thermocoupleattached to the samples. A Pace Scientific Pocket Logger Model XR340records time versus temperature. Each coat is considered dry when itstemperature is two degrees from ambient. Ambient temperature is 70°F.±5° F., and relative humidity is about 30%±5%. The results are shownin Table 2. As shown in Table 2, back-up coats formed from refractoryback-up slurries which use Megasol™ at a solids content of 50% dry inabout 67% of the time required to dry the five back-up coats formed fromrefractory back-up coat slurries which use NYACOL 830.

TABLE 2 Megasol ™ Binder NYACOL 830 Solids Binder Solids MOR- ExampleContent % Content % Back up Stucco MOR-green fired¹ 1* 50 — Fused Silica709 PSI 1334 PSI 30-50 mesh 2* 45 — Fused Silica 957 1474 30-50 mesh 3*40 — Fused Silica 834 1230 30-50 mesh 4* 35 — Fused Silica 719 123130-50 mesh 5* 50 — Mulgrain M47-22S 800  700 20-50 mesh 6* 45 — MulgrainM47-22S 935  958 20-50 mesh 7* 40 — Mulgrain M47-22S 923  856 20-50 mesh 8** — 30 Fused Silica 487  754 30-50 mesh  9** — 24 Fused Silica 605 550 30-50 mesh 10** — 30 Mulgrain M47-22S 470  616 20-50 mesh 11** — 27Mulgrain M47-225 640  627 20-50 mesh 12** — 24 Mulgrain M47-22S 658  57120-50 mesh ^(1.)Fired Modulus of Rupture is obtained after firing theshell at 1800° F. for 1 hour. *Total Drying time for 5 Back up coats is141 minutes **Total Drying time for 5 Back up coats is 236 minutes

COMPARATIVE EXAMPLES 13-18

These examples illustrate the increased strengths of the ceramic shellmolds due to use of refractory slurries which use Megasol™ over ceramicshells made from refractory slurries which use silica sols having anaverage particle sizes of 14 nanometers and 20 nanometers. The resultsare shown in Table 3.

EXAMPLE 13

The procedure of example 1 is followed except that Ludox® HS 40colloidal silica sol having an average particle size of 14 nanometer anda colloidal silica solids content of 35% is substituted for the Megasol™having 50% solids content. Ludox® HS 40 is available from E. I. DuPontdeNemours, Inc. The green and fired MORs are measured as in example 1.

EXAMPLE 14

The procedure of example 13 is followed except that the Ludox® HS 40colloidal silica sol has a colloidal silica solids content of 40%. Thegreen and fired MORs are measured as in example 1.

EXAMPLE 15

The procedure of example 1 is followed except that Ludox® TM colloidalsilica sol having an average particle size of 20 nanometer and acolloidal silica solids content of 35% is substituted for the Megasol™having 50% solids content. Ludox® HS 40 is available from E. I. DuPontdeNemours, Inc. The green and fired MORs are measured as in example 1.

EXAMPLE 16

The procedure of example 14 is followed except that the Ludox® TMcolloidal silica sol has a colloidal silica solids content of 40%. Thegreen and fired MORs are measured as in example 1.

EXAMPLE 17

The procedure of example 1 is followed except that the Megasol™colloidal silica sol has 35% solids content. The green and fired MORsare measured as in example 1.

EXAMPLE 18

The procedure of example 1 is followed except that the Megasol™colloidal silica sol has 40% solids content. The green and fired MORsare measured as in example 1.

TABLE 3 Megasol ™ Ludox ® HS 40 Ludox ® TM 40 Binder Solids BinderSolids Binder Solids Example Content % Content % Content % MOR-greenMOR-fired 13 — 35 — 350 PSI  325 PSI 14 — 40 — 250  230 15 — — 35 400 780 16 — — 40 325  600 17 35 — — 650 1015 18 40 — — 820 1535

In yet another embodiment of the invention, as illustrated bynon-limiting examples 19-20, potassium silicate is admixed with Megasol.The blend of potassium silicate and Megasol is present in at least oneof the prime coat and back-up coat. Preferably, the blend of potassiumsilicate and Megasol is present in both the prime coat composition andthe back-up coat composition. In each of the prime coat and back-upcoat, the potassium silicate may be present in an amount of up to 50% byweight of the Megasol. Preferably, the potassium silicate is present inan amount of about 6-8% by weight of the Megasol, most preferably about6%.

EXAMPLE 19

The procedure of example 1 is followed except that the refractory slurryused for both prime and back-up coats has the composition shown in Table4. The Megasol employed in Table 4 has a solids content of 40%.

TABLE 4 MATERIAL AMOUNT Megasol ™¹ 700 gm 400 mesh silica² 1375 gm PS9400 Surfactant³ 2 ml Potassium silicate⁴ 16.8 gm ^(1.)Megasol ™colloidal silica sol binder having 40% solids content from Wesbond Corp.^(2.)Fused silica ^(3.)Surfactant available from Buntrock Industries,Williamsburg, VA ^(4.)Kasil Potassium silicate from PQ Corporation.Weight ratio of SiO₂/K₂O is 2.5, 8.3% K₂O, 20.8% SiO₂, and 29.1% solids.

The green and fired MORs are measured as in example 1. The green MOR is913 psi. The fired MOR is 1424 psi.

EXAMPLE 20

The procedure of example 19 is followed except that the prime andback-up coats has the composition shown in Table 5.

TABLE 5 MATERIAL AMOUNT Megasol ™¹ 700 gm 140 mesh silica² 1375 gm PS9400 Surfactant³ 2 ml Potassium silicate⁴ 22.4 gm ^(1.)Megasol ™colloidal silica sol binder having 40% solids content from Wesbond Corp.^(2.)Fused silica ^(3.)Surfactant available from Buntrock Industries,Williamsburg, VA ^(4.)Kasil Potassium silicate from PQ Corporation.Weight ratio of SiO₂/K₂O is 2.5, 8.3% K₂O, 20.8% SiO₂, and 29.1% solids.

The green and fired MORs are measured as in example 1. The green MOR is912 psi. The fired MOR is 1362 psi.

In yet another embodiment of the invention, as illustrated innon-limiting examples 21-24, a commercial small size colloidal silicasol is admixed with Megasol. The blend of colloidal silica sol andMegasol is present in at least one of the prime coat and back-up coat.Preferably, the blend of colloidal silica sol and Megasol is present inboth the prime coat composition and the back-up coat composition. Ineach of the prime coat and back-up coat, the commercial small sizecolloidal silica sol may be present in the blend in an amount of up toabout 18% to about 85% by weight of the Megasol.

An especially useful commercial small size colloidal silica sol whichmay be admixed with Megasol as described above is NYACOL 830 having anaverage particle size of 8 nanometer and 24% silica solids from EKAChemicals Co. Other useful commercial small size silica sols which maybe admixed with Megasol according to this embodiment may have averageparticle sizes of about 12 nanometer, 14 nanometer, 20 nanometer and 22nanometer.

EXAMPLE 21

The procedure of example 1 is followed except that the refractory slurryused for both prime and back-up coats has the composition shown in Table6. The Megasol employed in Table 6 has a solids content of 50%.

TABLE 6 MATERIAL AMOUNT Megasol ™¹ 100 gm Tecosil 120F² 1190 gm Zircon325³ 330 gm PS 9400 Surfactant⁴ 2 ml NYACOL 830⁵ 562 gm ^(1.)Megasol ™colloidal silica sol binder having 50% solids content from Wesbond Corp.^(2.)Fused silica ^(3.)Calcined Florida Zircon, particle size of −325mesh from Continental Minerals ^(4.)Surfactant available from BuntrockIndustries, Williamsburg, VA ^(5.)NYACOL 830 @ 24% silica solids fromEKA Chemicals Co.

The green and fired MORs are measured as in example 1. The green MOR is740 psi. The fired MOR is 1618 psi.

EXAMPLE 22

The procedure of example 21 is followed except that Megasol™ is presentin an amount of 172 gms and NYACOL 830 is present in an amount of 490gms.

The green and fired MORs are measured as in example 1. The green MOR is870 psi. The fired MOR is 1493 psi.

EXAMPLE 23

The procedure of example 21 is followed except that Megasol™ is presentin an amount of 542 gms and NYACOL 830 is present in an amount of 120gms.

The green and fired MORs are measured as in example 1. The green MOR is858 psi. The fired MOR is 1668 psi.

In yet another embodiment of the invention, as illustrated in examples24-26, a colloidal binder that employs potassium silicate, a commercialsmall size colloidal silica sol and Megasol™ is used in at least one ofthe prime coat and back-up coats. Preferably, the colloidal binder ispresent in both the prime coat and the back-up coat. In each of theprime coat and back-up coat, the potassium silicate, small sizecolloidal silica sol, and Megasol™ may be present in the colloidalbinder in varying amounts. Megasol™ may be present in the colloidalbinder of this embodiment in an amount of up to about 10% to about 87%by weight of the colloidal binder; potassium silicate may be present inan amount of up to about 3% to about 8% by weight of the colloidalbinder; and the small particle size colloidal sol may be present in thecolloidal binder an amount of up to about 5% to about 87% by weight ofthe colloidal binder. In this embodiment, the potassium silicatepreferably is Kasil Potassium silicate from PQ Corporation. KasilPotassium silicate has a weight ratio of SiO₂/K₂O is 2.5, 8.3% K₂O,20.8% SiO₂, and 29.1% solids. Also, in this embodiment, the preferredsmall size particle size colloidal sol is NYACOL 830 having an averageparticle size of 8 nanometer and 24% silica solids from EKA ChemicalsCo.

EXAMPLE 24

The procedure of example 1 is followed except that the refractory slurryused for both prime and back-up coats has the composition shown in Table7.

TABLE 7 MATERIAL AMOUNT Colloidal Binder¹ 1000 gm Tecosil 120F² 1500 gmZircon 325³ 400 gm PS 9400 Surfactant⁴ 2 ml ^(1.)Blend of Megasol ™having 50% solids content from Wesbond Corp., Kasil Potassium silicateand NYACOL 830 wherein Megsol ™ is present in an amount of 87% by weightof the colloidal binder, Kasil is present in an amount of about 8% byweight of the colloidal binder, and NYACOL 830 is present in an amountof about 5% by weight of the colloidal binder. ^(2.)Fused silica fromC-E Minerals, particle size of 44-177 microns ^(3.)Calcined FloridaZircon, particle size of −325 mesh from Continental Minerals^(4.)Surfactant available from Buntrock Industries, Williamsburg, VA

EXAMPLE 25

The procedure of example 24 is followed except that in the colloidalbinder the Megsol™ is present in an amount of 10% by weight of thecolloidal binder, Kasil is present in an amount of about 3% by weight ofthe colloidal binder, and NYACOL 830 is present in an amount of about87% by weight of the colloidal binder.

EXAMPLE 26

The procedure of example 24 is followed except that in the colloidalbinder the Megsol™ is present in an amount of 57% by weight of thecolloidal binder, Kasil is present in an amount of about 5% by weight ofthe colloidal binder, and NYACOL 830 is present in an amount of about38% by weight of the colloidal binder.

What is claimed is:
 1. A method of manufacture of a ceramic shell moldcomprising, applying a coating of a prime coat slurry comprisingrefractory material and a colloidal silica sol onto an expendablepattern of thermoplastic material to produce a prime coated preform,drying said prime coated preform, applying at least one coating of arefractory back-up coat slurry comprising refractory material and acolloidal silica sol binder onto said prime coated preform to produce arefractory back-up coated preform, drying said refractory back-up coatedpreform, removing said thermoplastic pattern from said refractory backupcoated preform to produce a green shell mold, wherein in at least one ofsaid prime coat slurry and said refractory back-up coat slurry saidcolloidal sol is a blend of a large particle size aqueous colloidalsilica sol having an average particle size of about 40 nanometer, asmall particle size aqueous colloidal silica sol having an averageparticle size of about 8 nanometer, and potassium silicate.
 2. Themethod of claim 1 further comprising applying stucco material to atleast one of said prime coated preform or refractory back-up coatedpreform prior to drying of said prime coated preform or refractoryback-up coated preform.
 3. The method of claim 2 wherein said largeparticle size aqueous colloidal silica sol having an average particlesize of about 40 nanometer is present in an amount of about 10% to about87% by weight of said blend, and the potassium silicate is present in anamount of about 3% to about 8% by weight of said blend, and the smallparticle size aqueous colloidal silica sol having an average particlesize of about 8 nanometer is present in an amount of about 5% to about87% by weight of said blend.
 4. The method of claim 1, wherein saidlarge particle size aqueous colloidal silica sol has a solids content ofabout 40% to about 50%.
 5. The method of claim 1 wherein the silica solhas a basic pH.
 6. The method of claim 1 wherein the large particle sizesol and the small particle size sol are present in a ratio of 87 to 5.7. The method of claim 1 wherein the large particle size sol, the smallparticle size sol, and the potassium silicate are present in a ratio of87 to 5 to 8 respectively.
 8. A ceramic shell mold made by applying acoating of a prime coat slurry comprising refractory material and acolloidal silica sol onto an expendable pattern of thermoplasticmaterial to produce a prime coated preform, drying said prime coatedpreform, applying at least one coating of a refractory back-up coatslurry comprising refractory material and a colloidal silica sol binderonto said prime coated preform to produce a refractory back-up coatedpreform, drying said refractory back-up coated preform, removing saidthermoplastic pattern from said refractory backup coated preform toproduce a green shell mold, wherein in at least one of said prime coatslurry and said refractory back-up coat slurry said colloidal sol is ablend of a large particle size aqueous colloidal silica sol having anaverage particle size of about 40 nanometer, a small particle sizeaqueous colloidal silica sol having an average particle size of about 8nanometer, and potassium silicate, and applying stucco material to atleast one of said prime coated preform or refractory back-up coatedpreform prior to drying of said prime coated preform or refractoryback-up coated preform.
 9. The mold of claim 8 wherein the silica solhas a basic pH.